The present invention relates to new aqueous coating compositions and more particularly to new aqueous coating compositions having excellent corrosion- and water-resistant properties.
Water based paints used in electrodeposition coating, brush painting, spray coating and the like are becoming increasingly popular in the market because such paints are safe and economical and are capable of forming smooth and uniform films. It has heretofore been known to use natural drying oils such as tung oil, linseed oil, alkyd resins, epoxy-ester resins, etc. as starting polymers for vehicles of these water based paints. With more recent technical developments, it has been found that natural drying oils may be substituted by liquid polybutadienes such as a liquid polybutadiene containing 80% or more 1,2-bonds (Japanese Patent Gazette No. 46-954), a liquid polybutadiene containing 50% or more, especially 75-85%, 1,4-bonds (U.K. Pat. No. 1,102,652) and a liquid polybutadiene containing 50% or more cis-1,4 bonds (Japanese Patent Gazette No. 50-1580), which are reacted with an .alpha.,.beta.-unsaturated dicarboxylic compound, typically maleic anhydride, and then at least partially neutralized with a basic compound to thereby make the addition product soluble or dispersible in water.
These compositions, however, have both merits and disadvantages in the resultant film properties. For example, when a polybutadiene which contains predominantly 1,2-bonds is used as a starting material for such a composition, the composition forms films of high hardness and resistance to water, but of low resistance to corrosion and of such a high viscosity that there are practical difficulties in workability. When a polybutadiene which contains predominantly cis-1,4-bonds is used as a starting polymer of such a composition, such coating composition will offer films of high resistance to corrosion but of slightly lower hardness and water resistance. Therefore, attempts have been made to overcome said respective disadvantages by using suitable proportions of a mixture of such polybutadienes. As one example, a technique has been developed in which a polybutadiene which contains predominantly 1,2-bonds is used in combination with a second polybutadiene which contains predominantly 1,4-bonds, e.g. cis-1,4-bonds (Japanese provisional Patent Publication No. 48-25732). This technique provides a coating composition yielding films which are improved in water resistance compared with the films obtained when the cis-1,4-polybutadiene component is used alone, and which are improved in corrosion resistance compared with the films obtained when the 1,2-polybutadiene component is used alone. However, the films so obtained show only intermediate properties of both polybutadienes. It has heretofore remained impossible to obtain films which simultaneously possess both the high resistance to corrosion achieved when the cis-1,4-polybutadiene is used alone and the high resistance to water achieved when the 1,2-polybutadiene is used alone.